This adds support for the [text range selector](https://lottiefiles.github.io/lottie-docs/text/#text-range-selector) type, which allows a [text style](https://lottiefiles.github.io/lottie-docs/text/#text-style) to be applied to a certain range of the text, defined by a start, end, and offset.

The text range selector implementation currently has the following limitations:
- Only text layers drawn using a Font are supported. Adding support for the glyph draw path may be possible, but I don't have a sample Lottie file.
- Only one text range is currently supported per text layer, as the parser [drops all other entries in the array](https://github.com/airbnb/lottie-android/blob/c4cb2254eca3c70199f1de5e39e3872c8c42e473/lottie/src/main/java/com/airbnb/lottie/parser/LayerParser.java#L194-L196).
- Only index-based ranges are supported - percentage-based ranges are also allowed in the spec.
- Only ranges based on characters are supported. The [spec](https://lottiefiles.github.io/lottie-docs/constants/#text-based) allows characters, characters excluding spaces, words, and lines.
- Other options like easing (allows styling characters that are partially inside the range), randomize, and [shape](https://lottiefiles.github.io/lottie-docs/constants/#text-shape) of the range are not currently supported.

This also adds support for the opacity as an animatable text property which is applied multiplicatively with the transform opacity and the parent's alpha.

Also adds support for ranges specified as a percentage. Note that the provided sample has easing values included, which we don't support yet.